Recovery of hydrocarbons



United States Patent 3,108,946 RECGVERY 0F HYDRGCARBONS Earle C. Makin, In, El Dorado, Ark., assignor to Monsauto Chemical Company, St. Louis, Mo a corporation of Delaware Filed June 3, 1959, Ser. No. 317,856 6 Qlaims. (ill. 260-67) This invention relates to the thermal cracking of hydrocarbons. More particularly, this invention relates to the recovery of hydrocarbons from hydrocarbon mixtures obtained by the thermal cracking of hydrocarbons.

The thermal cracking of hydrocarbons is well known in the petroleum refining and petrochemical art. Various feed stocks can and have been used in such processes. These feed stocks include natural gasoline, propane, butane, kerosene, condensates, pent-anes, etc. All of these thermal cracking processes result in a cracked gas comprising a mixture of hydrogen, acyclic hydrocarbons, alicyclic hydrocarbons and aromatic hydrocarbons. This cracked gas is then subjected to various types of separation operations to recover pure components and specific mixtures of components. Because of the difficulty of separations, many pure components have not been recovered from these cracked gases for their chemical values but have been used for their energy value in mixtures with other components in the cracked gas. The advances of the petrochemical industry during the past decade have focused more and more attention on the desirability of recovering pure compounds for their chemical value from complex cracked gas mixtures. A particular troublesome separation has revolved about the aromatic content of the cracked gas mixture which at times can be high enough to be most significant. In most thermal cracking processes heretofore employed the aromatic content of the cracked gas has been removed from the cracked gas as a hydrocarbon mixture of aromatic hydrocarbons, acyclic hydrocarbons and alicyclic hydrocarbons, all containing six carbon atoms. Frequently this mixture contains as much as 35 weight percent of benzene. Because of the close boiling points of the various constituents of this C hydrocarbon stream, separation of the individual components by simple fractionating is virtually an impossibility. The aromatics can be separated by solvent extraction, but this is an expensive procedure. Furthermore, it is extremely difficult to separate cyclic olefins and diolefins from aromatics by solvent extraction techniques. Consequently, in the past this C hydrocarbon fraction has had little value other than its energy value and the more desirable chemical values of this stream have been lost.

It is an object of this invention to provide an improved process for the recovery of hydrocarbons produced by the thermal cracking of hydrocarbons. It is a particular object of this invention to provide an improved process for the recovery of C hydrocarbons produced in the thermal cracking of hydrocarbons. Additional objects will become apparent from the description of this invention.

In all thermal cracking processes utilizing a hydrocarbon feed stock, considerable quantities of hydrogen are produced. The separation of chemically pure hydrogen from the reaction mixture is not generally economically feasible and it is most frequently removed in a mixture with lower alkanes, such as methane, and the mixture used as a fuel gas. It has now been discovered that the chemical 'value of the C hydrocarbon mixture separated from the cracked gas obtained in the thermal cracking of hydrocarbons can be fully realized by substantially completely hydrogenating said C hydrocarbon stream in the presence of a hydrogenation catalyst with the fuel gas mixture comprising essentially hydrogen and methane 3,108,946 Patented Get. 29, 1963 "ice also obtained in the process, and then separating therefrom by simple fractional distillation substantially pure cyclohexane. Thus, the chemical value of the aromatics produced in the thermal cracking of hydrocarbons and the chemical value of hydrogen produced in the thermal cracking of hydrocarbons are fully recovered through the conversion of the aromatics to alicyclic hydrocarbons.

In some instances, depending upon the feed stock employed the C hydrocarbon stream contains sufiicient quantities of sulfur which should be removed prior to hydrogenation. 'In such instances the sulfur containing C hydrocarbon stream is catalytically desulfurized in the presence of a dehydrosulfurization catalyst using again as the hydrogen source the fuel gas stream comprising essentially hydrogen and methane obtained as a by-product in the thermal cracking of hydrocarbons;

While the process of this invention is applicable to the thermal cracking of any hydrocarbon, it is particularly applicable to the thermal cracking of natural gasoline, propane, butane, kerosene, condensates, pentanes, etc. for the purpose of producing ethylene as the primary product. The FIGURE is a schematic drawing of such an ethylene process embodying the novel improvement of this invention. The following specific example illustrates this process:

Normal butane from storage 1 is passed to vaporizer 2 Where it is heated to 500 F. and then mixed with steam in pipe 3 together with a C hydrocarbon recycle 4. This steam-hydrocarbon cracking feedstock is then introduced into the cracking furnace 5 where it is subjected to deep thermal cracking. The cracked gas leaves the cracking furnace via line 6 at a temperature of approximately 1500" F. and is introduced into quencher 7 where it is quenched by means of a Water-oil quench medium introduced into the top of the quencher via line 3. The cracked gas emerging from the top of the quencher is further cooled to a temperature of approximately F. and has the following approximate composition, percentages expressed as mol percent:

Perent H 13 CH 28 0 1-1 28 C H 9 C H l4 1 C hydrocarbons 6 C hydrocarbons and higher 1 The cracked gas is then introduced into topping column 10 and hydrogen and hydrocarbons containing from 1 to 3 carbon atoms and a minor amount of hydrocarbons containing 4 carbon atoms are taken overhead at a head temperature of approximately 50 F. and a hydrocarbon mixture containing 4 or more carbon atoms is removed as a bottom stream at a tempertaure of approximately 225 F. The overhead from the topping column is passed through caustic scrubber 11 and activated alumina dryer 12 to remove carbon dioxide and water and then introduced into depropanizing column 13. In this column hydrogen and hydrocarbons containing from 1 to 3 carbon atoms are removed overhead at a temperature of 'approximtaely 10 F. and a bottom stream composed of hydrogen and hydrocarbons containing a minor amount of hydrocarbons containing from 1 to 3 carbon atoms and substantially all of the C hydrocarbons introduced into the column is removed and returned to the topping column as reflux 14. The overhead from the depropanizing column is then introduced into the demethanizing column 15. In this column a fuel gas mixture composed removed overhead at a pressure of about 435 p.s.i.g. A mixture of C hydrocarbons and a minor amount of C hydrocarbons is removed as bottoms from this column and introduced into the deethanizer column 16. In this column all C hydrocarbons are taken overhead and the C hydrocarbons are removed as bottoms. The overhead from the deethanizing column is introduced into the ethylene column 17 where ethylene is taken overhead and ethane is removed as bottoms from the column and recycled to the cracking step as recycle stock 4.

The bottoms from topping column 10 are introduced to debutanizing column 18 and a 0., hydrocarbon fraction taken overhead and a bottom stream removed containing C and heavier hydrocarbons. This bottoms stream is commonly referred to as the aromatic oil and contains approximately 35% benzene. The aromatic oil is then introduced into depentanizing column 1? wherein C hydrocarbons are removed overhead and C and heavier hydrocarbons removed as bottoms. This bottoms stream is then introduced into column 20 wherein a C hydrocarbon fraction is taken overhead and all heavier hydrocarbons are removed as bottoms. The C hydrocarbon fraction has the following approximate composition, all percentages being expressed as percent by weight:

Percent Methylcyclopentadiene 4 1,3-cyclohexadiene l Cyclohexene 1 Methylcyclopentene 1 Hexenes-l-hexadienes 2 Benzene 92 Percent 2,3-dimethylbutane and Z-methylpentane 2 3-methylpentane 1 n-Hexane 2 Methylcyclopentane 4 Cyclohexane 91 Benzene 1 This hydrogenated stream is then introduced into final product column 23 wherein methylcyclopentane and lighter hydrocarbons are removed overhead at a temperature of approximately 72 C. and substantially pure cyclohexane is removed from the bottom of the column. Thus a convenient, effective and economic recovery of C hydrocarbons resulting from the cracking of hydrocarbons to produce ethylene has been effected.

At times the aromatic oil may contain significant amounts of sulfur, dictating the advisability of desulfurizing the stream prior to hydrogenation. Insuch cases, the C hydrocarbon fraction taken overhead from column 20 is then introduced into desulfurizer 24 where it is desulfurized at a pressure of about 500 p.s.i.g. at a temperature of 600 F. using again as the hydrogen source, the overhead stream 22 from demethanizin-g column 15. The desulfurizcd product is then introduced into hydrogenator 21 where it is hydrogenated as hereinbefore described and the cyclohexane then separated by distillation. It will be obvious to those skilled in the art that the operating conditions of the various unit operations carried out in this process can be varied substantially, de-' pending upon the particular circumstances presented by the commercial installation desired. ,Any of the conventional thermal cracking furnaces can be used in this process. The feed stock can be varied. In addition to normal butane, pentanes, natural gasoline, kerosene, condensates, and various other hydrocarbon fractions can be utilized as feed to the cracking furnaces. In the various rectification steps, the design and operating conditions of the rectification columns are not critical factors, as long as thementio'ned separations are obtained. Bubble cap, perforated tray and packed columns are all suitable in this operation.

The hydrogenation step of this process can be carried out in accordance with any of the well known techniques. Nickel, palladium, platinum, and so forth are all suitable catalysts in this operation. Hydrogenation pressure can be varied from 400 to 2000 p.s.i.g. and the hydrogenation can, depending upon the catalyst utilized, be carried out at temperatures ranging from room temperature to 500 F. or higher.

Similarly, the desulfurization step, if this step is advisible under the particular circumstances, can be carried and using any of the known methods with which those skilled in the art are acquainted. Cobalt molybdenum sulfide, tungsten-nickel sulfide, nickel sulfide, and so forth are all suitable catalysts and desulfurization can be car ried out at pressures ranging from 100 p.s.i.g. up to 2000 p.s.i.g. or even higher. The temperature can be varied from room temperature to 900 F. or higher if desired.

What is claimed is:

1. In a process for the thermal cracking of hydrocarbons wherein there is separated from the cracked gas mixture a fuel gas comprising a mixture of substantial quantities of hydrogen and lower hydrocarbons and an aromatic oil comprising a mixture of substantial quantities of aliphatic and aromatic hydrocarbons which hydrocarbons are ditficultly separable from the mixture by fractional distillation because of the close boiling points of the hydrocarbons contained in the mixture, the improvement which comprises hydrogenating said aromatic oil by contacting said aromatic oil with said fuel gas in the presence of a hydrogenation catalyst to produce a mixture consisting essentially of saturated acyclic and saturated alicyclic hydrocarbons and separating therefrom by fractional distillation substantially pure alicyclic hydrocarbons.

2. In a process for the thermal cracking of hydrocarbons wherein there is separated from the cracked gas mixture a fuel gas comprising a mixture of substantial quantities of hydrogen and lower hydrocarbons and an aromatic oil comprising a sulfur-containing mixture of substantial quantities of aliphatic and aromatic hydrocarbons which hydrocarbons are difficultly separable from the mixture by fraction distillation because of the close boiling points of the hydrocarbons contained in the mixture, the improvement which comprises desulfurizing said aromatic oil by contacting said aromatic oil With said fuel gas in the presence of a dehydrosulfurization catalyst to produce a sulfur-free aromatic oil, hydrogenating said sulfur free aromatic oil by contacting said sulfur-free aromatic oiiwith said fuel gas in the presence of a hydrogenation catalyst to produce a mixture consisting essentially of saturated acyclic and saturated alicyclic hydrocarbons and separating therefrom by fractional distillation substantially pure alicyclic hydrocarbons.

3. In a process for the thermal cracking of hydrocarbons wherein there is separated from the cracked gas mixture a fuel gas comprisin a mixture of substantial quantities of hydrogen and lower hydrocarbons and an aromatic oil comprising a mixture of substantial quantities of aliphatic and aromatic hydrocarbons all containing, 6 carbon atoms which hydrocarbons are difiicultly sop-- arable from the mixture by fractional distillation because of the close boiling points of the hydrocarbons contained in the mixture, the improvement which comprises hydrogenating said aromatic oil by contacting said aromatic oil with said fuel gas in the presence of a hydrogenation catalyst to produce a mixture consisting essentially of saturated acyclic and saturated alicyclic hydrocarbons and separating therefrom by fractional distillation substantially pure alicyclic hydrocarbons.

4. In a process for the thermal cracking of hydrocarbons wherein there is separated from the cracked Igas mixture a fuel gas comprising a mixture of substantial quantities of hydrogen and lower hydrocarbons and an aromatic oil comprising a sulfur-containing mixture of substantial quantities of aliphatic and aromatic hydrocarbons all containing 6 carbon atoms which hydrocarbons are diflicultly separable from the mixture by fractional distillation because of the close boiling points of the hydrocarbons contained in the mixture, the improvement which comprises desulfurizing said aromatic oil by contacting said aromatic oil with said fuel gas in the presence of a dehydrosul'furization catalyst to produce a sulfur-free aromatic oil, hydrogenating said sulfur-free aromatic oil by contacting said sulfur-free aromatic oil with said fuel gas in the presence of a hydrogenation catalyst to produce a mixture consisting essentially of saturated acyclic and saturated alicyclic hydrocarbons and separating therefrom by fractional distillation substantially pure alicyclic hydrocarbons.

5. [[n a process for the thermal cracking of hydrocarbons wherein there is separated from the cracked gas mixture a fuel gas comprising a mixture of substantial quantities of hydrogen and lower hydrocarbons and an aromatic oil comprising a mixture of substantial quantities of aliphatic hydrocarbons containing 6 carbon atoms and benzene, the improvement which comprises hydrogenatinig said aromatic oil by contacting said aromatic oil with said fuel gas in the presence of a hydrogenation catalyst to produce a mixture consisting essentially of saturated acyclic hydrocarbons and cyclohexane and separating therefrom by fractional distillation substantiall pure cyclohexane.

6. In a process for the thermal cracking of hydrocarbons wherein there is separated from the cracked gas mixture a fuel gas comprising a mixture of substantial quantities of hydrogen and lower hydrocarbons and an arobons and cyclohexane and separating therefrom by fractional distillation substantially pure cyclohexane.

References Cited in the file of this patent UNITED STATES PATENTS 2,276,081 McGreW Mar. 10, 1942 2,276,103 Seguy Mar. 10, 1942 2,328,828 Marschner Sept. 7, 1943 2,382,910 Pinkston Aug. 14, 1945 2,398,846 Munday Apr. 23, 1946 2,671,754 De Ro-sset et a1 Mar. 9, 1954 2,852,440 Smith et al. Sept. 16, 1958 2,884,469 McCaulay Apr. 28, 1959 2,906,784 Dean et al. Sept. 29, 1959 

1. IN A PROCESS FOR THE THERMAL CRACKING OF HYDROCARBONS WHEREIN THERE IS SEPARATED FROM THE CRACKED GAS MIXTURE A FUEL GAS COMPRISING A MIXTURE OF SUBSTANTIAL QUANTITIES OF HYDROGEN AND LOWER HYDROCARBONS AND AN AROMATIC OIL COMPRISING A MIXTURE OF SUBSTANTIAL QUANTITIES OF ALIPHATIC AND AROMATIC HYDROCARBONS WHICH HYDROCARBONS ARE DIFFICULTY SEPARABLE FROM THE MIXTURE BY FRACTIONAL DISTILLATION BECAUSE THE CLOSE BOILING POINTS OF THE HYDROCARBONS CONTAINED IN THE MIXTURE, THE IMPROVEMENT WHICH COMPRISES HYDROGENATING SAID AROMATIC OIL BY CONTACTING SAID AROMATIC OIL WITH SAID FUEL GAS IN THE PRESENCE OF A HYDROGENATION CATALYST TO PRODUCE A MIXPRESENCE OF A HYDROGENATION CATALYST TO PRODUCE A MIXTURE CONSISTING ESSENTIALLY OF SATURATED ACYCLIC AND SATURATED ALICYCLIC HYDROCARBONS AND SEPRATING THEREFROM BY FRACTIONAL DISTILLATION SUBSTANTIALLY PURE ALICYCLIC HYDROFRACTIONAL DISTILLATION SUBSTANTIALLY PURE ALICYCLIC HYDROCARBONS. 